1. Field of the Invention
This invention relates to the field of channel filters for use in detecting and decoding read signals from mass storage media.
2. Background Art
In present day data processing systems, it is desired to provide a large amount of memory which can be accessed in a minimum amount of time. One type of memory which has enjoyed widespread use in the data processing field is that of magnetic media disk memories.
In general, disk memories are characterized by the use of one or more magnetic media disks stacked on a spindle assembly and rotating at a high rate of speed. Each disk is divided into a plurality of concentric "tracks" with each track being an addressable area of the memory array. The individual tracks are accessed through magnetic "heads" which fly over the disks on a thin layer of air. Typically, the disks are two sided with a head accessing each side.
The heads are in substantial alignment and are mounted to an actuator motor which moves the heads from track to track during the reading and writing of information from the disks. The actuator motor may be a "voice coil" electrodynamic motor which has a coil moving within a permanent magnetic field, defining a cylindrical core. Alternatively, the motor may have a "rotary" type coil, such as is described in U.S. patent application Ser. No. 444,465 filed on Nov. 24, 1982, and 893,955 filed on Aug. 7, 1986, both assigned to the assignee of the present invention.
Information is encoded on magnetic media disks as a series of binary "bits" indicating a "1" or a "0". These bits are encoded as the presence or absence of a magnetic flux reversal. The capacity of a storage disk is thus dependent on the number of flux reversals which can be accurately written onto and read from a magnetic media storage disk.
In present day technology, the magnetic flux reversals are written onto, and read from, the magnetic media through the use of thin film heads. The heads have two thin pole faces which permit the application of flux reversals to the magnetic media. Currently, the widths of these pole faces are limited by deposition technology, resulting in time and economic limits on the width of the pole faces.
The finite pole tip width of present day thin film heads result in pre-cursive and post-cursive undershoots on the output signals (flux reversals) during the reading of information stored on a magnetic media disk. Pre-cursive and post-cursive undershoot can be limited or eliminated by utilizing wider pole faces on the magnetic head. However, as previously mentioned, present day manufacturing capabilities place severe time and economic limits on the width of the pole tips. For example, widening the pole faces requires additional deposition time.
The pre-cursive and post-cursive undershoots result in a widening of the "time window" which defines a "bit". Further barrowing of the pole faces of the magnetic head could be utilized to narrow the window of each pulse, resulting in the potential for greater density, but such a configuration results in a deepening of the pre-cursive and post-cursive undershoots. Further, if the pole faces of a magnetic head are too narrow, it is very difficult to maintain required accuracy in the manufacture of thin film head throat height. The result is that it is difficult to generate a magnetic field strong enough to erase data already written onto a magnetic media disk.
Still, it is desired to narrow the time window of signals read by the magnetic head so that more signals may be read in a given time and correspondingly greater information density may be achieved. One prior art attempt to narrow the time window associated with semi-infinite pole tips is known as a minimum bias windowing technique and is described in U.S. Pat. No. 4,344,093 issued Aug. 10, 1982 and assigned to the Sperry Corporation. In minimum bias windowing, read pulses are narrowed so that they do not overlap and thus do not cause peak shift or amplitude variation of the read signal. Pulse narrowing results in an increase in the bandwidth of the read pulses, requiring a corresponding increase in the read system bandwidth with an accompanying increase in noise. The prior art utilizes an equalizer circuit coupled to the read circuit to produce a symmetrical signal with a limited degradation of signal to noise ratio.
Although this film heads exhibit peaked, wide bandwidth frequency response plots, the use of conventional polynomial frequency response low pass filters results in large amounts of peak shift due to inter-symbol interference in connection with pre-cursive and post-cursive undershoots. These peak shifts cause significant reduction in the achievable window margin in magnetic media storage systems.
Therefore, it is an object of the present invention to provide a channel filter which substantially elminates the excessive inter-symbol interference characteristic of thin film heads in reading polarity reversals from magnetic media storage systems.
It is another object of the present invention to provide a channel filter in which a moderate order L-C filter may be utilized.
It is yet another object of the present invention to provide a channel filter resulting in improved recording preformance in terms of horizontal and vertical margins.